Freezing Valves?

Today, dealing with cryogenic temperatures is a common task for a fluid system designer. Many common fluids used in propulsion, including liquid oxygen and liquid nitrogen, fall under this category. While the principles behind the control of these fluids remain largely the same as for milder fluids, cryogenics present a unique series of problems to the system designer.

It is well known in the engineering profession that as temperature increases, materials tend to expand. The inverse is also true: as temperature decreases, most materials shrink. This can present a plethora of problems to fluid system and component designers. The most common problem in cryogenic fluid devices is leakage. The elastomeric materials typically used as seals have a much larger coefficient of thermal expansion than the other materials present in the valve. This causes the elastomers to shrink much faster than the valve geometry which surrounds them.

In a traditional sense, sealing off wetted areas of fluid control valves has been achieved by compression of a deformable material onto a hard surface, such as steel. As the pressure of the fluid increases, it is generally necessary to increase the amount of compression of the seal material to maintain contact with the sealing surface. Elastomeric materials, such as nitrile rubber, have been used extensively due to their ability to deform a great deal without any permanent damage. This property is particularly useful in the case of dynamic seals, where the amount of compression varies over many cycles of a valve’s operation.

Unfortunately, these materials do not behave as well under severe temperature conditions. As elastomers shrink due to cold temperatures, the compression they can apply to a sealing surface decreases due to the decrease in size of the seal. This reduction in compression is compounded by the change in elasticity of the material with the temperature. As temperature decreases, materials become more brittle, and less capable of elastic deformation. This effect is well documented, and can be quantified by the Charpy or Izod test methods.

A simple and effective method for dealing with the shrinkage and brittleness problems of cryogenic conditions is the use of springs to apply compressive forces with these seals. Springs can be used in sealing systems to apply a more constant compressive load on the seal material, which mitigates the effect of cryogenic shrinking on the seal.

Another method for effective cryogenic sealing is material selection. While all materials are susceptible to the change in properties due to thermal conditions as described above, some are better suited for severe conditions than others. For our cryogenic products, CEI exclusively uses polytetrafluoroethylene (PTFE), which is more commonly known as Teflon (1). Teflon is a non-reactive polymer with excellent thermal characteristics. The coefficient of thermal expansion for a typical elastomeric sealing compound, nitrile rubber, is 112 * 10 -6 /˚K, while the coefficient for Teflon is 41.6 * 10 -6 /˚K. This is slightly more than one third of the value for rubber, which translates to roughly one third the expansion/contraction of a given length of Teflon in comparison with the same length of rubber.

Metals can also be used as seals in cryogenic components, and in static conditions they can perform quite well. However, the large amount of force necessary to produce a seal with metal makes these seals undesirable for dynamic seals of moving components.